Embodiments of the present invention generally relate to an apparatus for processing substrates. More particularly, the invention relates to a processing platform for performing atomic layer deposition (ALD) and chemical vapor deposition (CVD) on substrates.
The process of forming semiconductor devices is commonly conducted in substrate processing platforms containing multiple chambers. In some instances, the purpose of a multi-chamber processing platform or cluster tool is to perform two or more processes on a substrate sequentially in a controlled environment. In other instances, however, a multiple chamber processing platform may only perform a single processing step on substrates; the additional chambers are intended to maximize the rate at which substrates are processed by the platform. In the latter case, the process performed on substrates is typically a batch process, wherein a relatively large number of substrates, e.g. 25 or 50, are processed in a given chamber simultaneously. Batch processing is especially beneficial for processes that are too time-consuming to be performed on individual substrates in an economically viable manner, such as for ALD processes and some chemical vapor deposition (CVD) processes.
The effectiveness of a substrate processing platform, or system, is often quantified by cost of ownership (COO). The COO, while influenced by many factors, is largely affected by the system throughput (i.e., the number of substrates processed per hour) and system footprint (i.e., the total floor space required to operate the system in a fabrication plant). Footprint typically includes access areas adjacent the system that are required for maintenance. Hence, although a substrate processing platform may be relatively small, if it requires access from all sides for operation and maintenance, the system's effective footprint may still be prohibitively large.
The semiconductor industry's tolerance for process variability continues to decrease as the size of semiconductor devices shrink. To meet these tighter process requirements, the industry has developed a host of new processes which meet the tighter process window requirements, but these processes often take a longer time to complete. For example, for forming a copper diffusion barrier layer conformally onto the surface of a high aspect ratio, 65 nm or smaller interconnect feature, it may be necessary to use an ALD process. ALD is a variant of CVD that demonstrates superior step coverage compared to CVD. ALD is based upon atomic layer epitaxy (ALE) that was originally employed to fabricate electroluminescent displays. ALD employs chemisorption to deposit a saturated monolayer of reactive precursor molecules on a substrate surface. This is achieved by cyclically alternating the pulsing of appropriate reactive precursors into a deposition chamber. Each injection of a reactive precursor is typically separated by an inert gas purge to provide a new atomic layer to previous deposited layers to form an uniform material layer on the surface of a substrate. Cycles of reactive precursor and inert purge gases are repeated to form the material layer to a desired thickness.
The biggest drawback with ALD techniques is that the deposition rate is much lower than typical CVD techniques by at least an order of magnitude. For example, some ALD processes can require a chamber processing time from about 10 to about 200 minutes to deposit a high quality layer on the surface of the substrate. In choosing such ALD and epitaxial processes for better device performance, the cost to fabricate devices in a conventional single substrate processing chamber would increase due to very low substrate processing throughput. Hence, when implementing such processes, a batch processing approach is needed to be economically feasible.
However, the quality of the films deposited by a batch processing system decrease as the throughput is increased. Often, better film quality is obtained by a single-wafer processing system. Therefore, there is a need for a ALD and CVD substrate processing chambers to maximize substrate processing throughput and minimize footprint, without sacrificing the production of high film quality.